It is often desirable to modify the surface interface of polymeric materials to produce a variety of related surface effects such improved slip or lubricity, reduced blocking, and to lubricate process equipment. It is also desirable to modify polymeric surfaces to improve release of adhesives and promote demolding or adhesion of other materials from a polymeric composition surface. It is also generally understood in the art that improved slip and reduced blocking are a function of reduced coefficient of friction (COF) for the particular composition. A variety of prior art additives and modifiers have been developed to attempt to provide these attributes to different types of polymeric materials. These additions and modifiers can be classified as anti-block additives, slip aids, COF modifiers and release aids, and have somewhat related but different purposes and attributes, as described below.
1. Anti-Block Additives
Polyolefin resins are regularly used to produce sheets and films. These sheets and films are stacked or rolled in storage and use. The sheets or rolls of film can adhere to each other and become very difficult to separate due to surface tackiness and other molecular binding mechanisms. This phenomenon is called blocking. Anti-blocking agents are commonly and interchangeably called slip aids. Anti-blocking agents are commonly added to the melt phase of the resins during compounding or extrusion to mitigate this blocking effect.
The most common approach to reducing blocking behavior is the use of certain particulates as additives that are dispersed in the resin. These anti-blocking additives take various forms such as Zeolite, inorganic silicates, silica or powdered silicone rubber. However, these dispersed solids can produce a strong haze effect and can produce unattractive films. Further, like other proposed solutions, this solution for blocking of these films can create problems with regard to reducing the COF of the film. For example, a polypropylene film is often laminated to other films. In particular, glassine paper is commonly laminated to polypropylene film. Such a laminate, when provided as a thin surface layer containing finely divided inorganic material, exhibits significantly higher COF values than does the unlaminated polypropylene film as a result of the particulate additive present in the polypropylene film. Such laminates are known to perform marginally at best on conventional form, fill and seal machines.
Another prior art attempt to reduce blocking is disclosed in U.S. Pat. No. 4,327,009 which describes the use of polyglycols and other low molecular weight additives to polyolefin films to reduce blocking. These additives are selected due to their propensity to migrate to the surface, and as such suffer from the limitations created by the migrating additives. The migration behavior of these or any other type of additive to a polymeric compound is a function of the solubility parameter of the additive in relation to that of the base polymer and the molecular weight of the additive. The more mismatched the solubility parameters and the lower the molecular weight, the faster the additive can migrate to the interface of the polymer and cause undesirable effects thereon, such as an increase of COF.
2. Slip Aid Additives
Slip aids also reduce blocking but further aid in lubricating handling and converting equipment down stream from extrusion or molding. Injection molded parts also benefit from slip aids when the slip aids are incorporated within the material formed into the parts as the parts have improved lubricity against each other when handled. This reduces marring and enables the material/parts to flow better when handled in bulk.
The lubricity effects discussed herein are to be distinguished from the industry term “internal lubricant” which is an additive class of materials that effectively “plasticizes” the polymer melt to improve processing and flow.
Many slip agents based on silicones and fatty amides, such as erucamide and oleamide, are well known in the art. Although they do reduce the coefficient of friction, lubricate and improve release, their effectiveness depends upon their ability to migrate to the surface of the film, which is required for these slip aids to be effective. In addition, the development of the desired level of anti-block or slip for the film composition in which the slip aids are utilized is strongly dependent upon the type slip aid used, the amount used, the storage time of the film and storage temperature. The heat history of the film while in storage, shipping and during subsequent converter processes also significantly affects the surface lubricity effect and is thus difficult to predict and control. In addition, the presence of fatty acid amides used as a slip aid that migrates onto the exterior of the film surface, while decreasing the COF of the film, unfortunately results in visible adverse appearance effects for the film, manifested by an increase in haze, a decrease in gloss and the presence of streaks. These fatty amide slip aid materials also adversely affect the adhesion of solvent and water-based inks, coatings and adhesives.
The migration of silicones utilized as slip aids can also cause similar downstream processing problems as the amides. Recent advances in silicone additive technology have been made to reduce the migration rates of the silicones and siloxanes. However, these modifications made to the silicone slip aids often cause the silicone slip aids to suffer from incompatibility with the resins which makes compounding very difficult. As a result, consistent compounding of these silicone materials is recognized in the industry as being very difficult to achieve. Commercial offerings of these materials can also still migrate to a small extent and continue to cause downstream issues due to this migration.
3. COF Modifiers
Polypropylene films have found wide acceptance in the packaging industry, especially as a food packaging agent, because of their superior physical properties. Polypropylene film, usually biaxially oriented, is characterized by high tensile modulus and stiffness, and excellent optical clarity and a certain degree of moisture resistance. Moreover, polypropylene film is highly pervious to gases and air. However, polypropylene possesses one major disadvantageous property, namely, a high inherent coefficient of friction (COF). This high COF significantly complicates the processing of the polypropylene film. In particular, the polypropylene film processing is impeded by poor transport, caused by its high COF, over rollers, guides and the like. In addition, the high COF creates film storage problems. In particular, because of its high COF one layer of polypropylene film sticks to those above and beneath it, creating the blocking effect referenced earlier.
This serious disadvantage of polypropylene film is well known to those skilled in the art. Thus, many proposed solutions to overcome this major deficiency have been proposed in the art. One such proposal has been to incorporate additives in the polypropylene resin processed to form the film. A favorite additive utilized for COF reduction are slip aids, specifically fatty acid amides. These amides decrease the COF of the polypropylene film as they migrate to the film surface after heat treatment and aging. Although this method has been used, actual COF is a function of the heat history to which the film has been exposed during shipping, storage and processing. As such, it is subject to wide variation. More significantly, the presence of fatty acid amides on the film surface oftentimes adversely affects the appearance of the film as manifested by decreased gloss and the presence of streaks, as discussed previously.
Another serious disadvantage of using fatty acid amides is the detrimental effect that the fatty acid amide additives have on polypropylene film surface wetability and adhesion when the additive has migrated onto the surface of the film to provide the COF reduction. This adverse adhesion characteristic caused by the additives applies to coating, inks, adhesives and the like, especially in water based forms that are applied to the surface of the films.
It is also known to coat polypropylene films with certain fatty acid amides to impart lubricating and anti-blocking characteristics, as opposed to incorporating the amines within the film as an additive. However, the application of such coatings by the film manufacturer is not particularly attractive because of the added equipment and corresponding expense of doing so, as well as the requirement that the amine coatings be applied as solutions in organic solvents. Health and safety factors dictate against the in-plant utilization of organic solvents in coating processes undertaken during the film manufacture.
Other slip additives have been suggested for use with thermoplastic films to overcome the inherent problem of high COF in thermoplastic films. One such solution, as disclosed by U.S. Pat. No. 4,302,506, is the use of a latex coating containing stearamidopropyl-dimethyl-beta-hydroxyethylammonium nitrate and a crosslinkable acrylic copolymer. However, this method requires post treatment of the film or article by applying the additive as a coating to the surface of the polymeric film. While avoiding the problems caused by having to utilize organic solutions in applying the coating, this alternative additive is still undesirable because it requires downstream coating and curing equipment, and the additional expense associated with them.
4. Internal Mold Release and Adhesive Release Aids
It is also desirable to incorporate internal mold release aids into thermoplastics and thermoset resin compositions because these release aids assist in demolding extruded or cured parts formed from the thermoplastic or thermoset resins from production molds and processing equipment. For example, U.S. Pat. No. 5,883,166 describes the use of a liquid mold release agent for unsaturated polyester resins. However, in general, liquids are not commercially feasible for use as release aids due to their low molecular weight which allows the additive to migrate out of the compound too quickly, subsequently causing a variety of problems with the molded product in commercial use, such as those described previously.
The above discussion reflects the need in the art for improved additives for polypropylene and other polyolefin films that are capable of creating films characterized by improved anti-blocking characteristics and decreased COF. The same desired effects provided by additives of this type should also extend to other polymeric materials, such as the materials utilized to form the molds in which products are formed from these polyolefin films, which could potentially be made from a polyolefin of other suitable compound including additives of this type, to improve the release of the product from the mold when a product formed from a polymeric material is molded or cured within or in proximity to the mold.
In addition, when providing these attributes, it is desirable to have an additive material that has minimal or no migration within the polymeric compound, such that the additive is considered essentially permanent, to avoid the problems associated with the migration of prior art additive onto the surface of the polymeric film or item. In addition to the non-migratory properties of the additive, it is incumbent that the particular additive or compound utilized for the improvement in film slip property also not correspond or create to any decline in any other properties of the film in which the additive is utilized, which is typical of the previous additive solutions advanced in the prior art.